We have found that Pseudomonas aeruginosa produces two distinct groups of exotoxins that are primarily responsible for the lethal effects of its infections. Materials of the first group, designated group A toxin, are sufficiently stable to be concentrated and purified with the usual biochemical means used for preparing exotoxins from other microorganisms. This group includes exotox that are immunologically identical but there are physical differences among the toxins that are produced by different strains of P. aeruginosa. Toxins from the second group, designated group B, are quite unstable and methods are still to be found which allow adequate concentration and purification of these toxins for the production of antitoxic sera. There is a possibility that this group will include antigenically different toxins. The objectives of the proposed research are to produce and purify both groups of exotoxins for characterization as well as production of antitoxic sera of sufficiently high titers for use in the prevention of the lethal effect of the infections caused by this organism in man. The final product, hopefully, will be a polyvalent antitoxic serum that covers all serotypes of the exotoxins produced by this species.